Glass has been widely used as a material for containers of seasonings, oils, juices, carbonated drinks, beer, Japanese sake, cosmetics, detergents and others. Glass containers, however, are usually recovered and recycled after use because they need high manufacturing costs. Glass containers are, moreover, so heavy as to require high transportation cost, and so fragile as to need careful handling.
In order to solve these problems of glass containers, a variety of plastic containers have recently come to be used rapidly in substitution for glass containers. As materials, various plastics are used depending on the type of contents and purpose of use. For example, polyethylene terephthalate and polyethylene naphthalate are employed for containers of juices, soft drinks, carbonated drinks, seasonings, detergents, cosmetics and others because they are superior in mechanical strength, heat resistance, transparency and gas barrier properties. In the above usage, blow molded containers to be filled with juices, soft drinks, and carbonated drinks are expected to be sterilized and filled with contents at high temperature, and therefore they are required to be made of resin having an excellent heat resistance capable of withstanding high-temperature filling. Such blow molded containers are, moreover, expected to have excellent transparency and form stability which brings less scattering of internal volume of the containers.
Conventionally known bottles made of polyethylene terephthalate or polyethylene naphthalate have high gas barrier properties and heat resistance, but bottles made of synthetic resin having further excellent transparency and heat resistance in addition to gas barrier properties are desired to be developed.
The inventors made every effort for obtaining bottles of synthetic resin having superior heat resistance and transparency as well as gas barrier property, and found out that gas barrier properties were extraordinarily improved in bottles made of polyethylene naphthalate resin wherein a stretch index defined as follows satisfied specific conditions, thereby accomplishing a first invention.
The inventors also found out that gas barrier properties were enhanced greatly in bottles made of polyethylene naphthalate resin, wherein permeability constant Pc to carbon dioxide gas defined as follows was not more than a specific value, and at the same time, the mean thickness constant Tc of body at intermediate part, which is defined below, was not more than a specific value, thereby reaching a second invention.
The inventors, moreover, found out that gas barrier properties were extremely superior and heat resistance was superior in stretched bottle made of polyethylene naphthalate resin wherein stretched polyethylene naphthalate resin at the intermediate part of the bottle body showed peak values at specific positions in an X-ray interference intensity distribution curve, thereby establishing a third invention.
Meanwhile, bottles made of polyethylene naphthalate resin or films made of polyethylene naphthalate resin are already known, for example, in Japanese Patent Publn. No. 49-22945, but the known bottles or films made of polyethylene naphthalate do not have such properties as defined in this invention, and polyethylene naphthalate resin bottles in which gas barrier properties are extraordinarily improved cannot be obtained unless such properties as defined in this invention are provided.